This is from memory and several updates ago but I remember Icedrake Fjord as Norsca (Wulfric) not having some of the SFO buildings.

I think that if a settlement has landmarks add by 'Landmarks of the Old World' it causes conflict with SFO's building changes. Ex: Icedrake Fjord is messed up but if I jump over to Konquata in Albion, everything's fine.

I suggest checking the list of settlements affected by 'Landmarks of the Old World' here:
https://docs.google.com/spreadsheets/d/128sCN-7PPvCejf81A3TB5qH2W01hg7RPwMbZxzpqgpI/edit#gid=1741720726

Might save you and us the trouble of checking every single settlement with every faction and race.

I really appreciate the effort you're putting into getting these mods to play nice. Thanks.

no buildings from SFO to Barak varr-dwarf

Bretonnia, Alberic de Bordeleaux: separated in 3 group to be more precise. Settlements like Couronne with the vanilla landmarks are not counted in. For my progress, I took everything west of Athel Loren, Marienburg and a bit of Middenland, everything South so Skryre, Sartosa and the 3 southern princes, just the settlement North of Arkhan in Nehekara.

Settlement without additionnal landmark and SFO not working : Matorca, The Black Pit,

Settlement with additionnal landmark and SFO not working : Castle Bastonne, Montfort, Aquitaine, Brionne, Al Haikk, L'Aiguille, Bilbali, Castle Artois, Gisoreux, Lyonesse, Quenelles, Myrmidens, Zvorak, Lashiek

Settlement with additionnal landmark and SFO working : Bordeleaux, Castle Carcasonne, Fyrus, Moussillon, The Moot, Marienburg.

PS: Question about "Markets of Helmgart" at Helmgart, it's a fort with just the building, no landmark but I don't think it's possible to add a building to fort/gate.

I also remember that Praag was SFO fine with Clan Angrund. The rest I can't remember.

When used in conjunction with SFO, there is a problem that additional buildings such as Wood Elf, Tomb King, and Nosca cannot be used. However, if compatible mode is used, CTD occurs and the game ends. https://steamcommunity.com/sharedfiles/filedetails/?id=2379693302
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Using the upper mode solves the problem of not being able to use SFO additional buildings, but CTD occurs when it is three turns over, perhaps because of a start-force collision.